Ladders are used for a wide variety of tasks that require ascending to a level normally out of reach when standing on the ground or floor. Most ladders used for ascending substantial heights, such as heights in excess of four to six feet, are designed to stand on the ground and lean against a flat surface, such as a roof or wall of a house. However, when such ladders are used to ascend trees, utility poles and other relatively small and/or non-flat surfaces, the ladder may tend to engage the non-flat surface along only one of the side rails of the ladder or along the top step thereof. As such, the ladder may tend to slide or twist as the climber ascends up the ladder or when the climber should lean outwardly to reach for something. Such ladders are typically stabilized by leaning the upper portion of the ladder against the object to be climbed and using a second person at the base to grasp and stabilize the upper portions of the ladder, while a first person climbs the ladder. The requirement for the second person adds a significant expense to the uses of existing ladders.
U.S. Pat. No. 4,946,004 of Henson discloses a ladder for gripping a pole and having a yoke member on which are pivotally mounted arcuate gripping arms about pivot pins. Each arm has an inner arm portion and is biased "open" by a spring. Engagement of the arm portions by the pole causes the arms to rotate to a point where the spring is disposed over the pivot pin and biases the arms to a closed position. No catch mechanism other than the spring is provided by Henson.
German Patent No. 903,147 shows a ladder mounted pole gripping device including jaws normally biased open by respective springs, and closed by a linkage, which is tied to a cord. The cord may be wound onto a reel disposed at the bottom of the ladder to pull the jaws to their closed positions. A detente engages the reel to prevent the reel from accidentally unwinding and releasing the jaws. The reel is unwound to release the jaws.
U.S. Pat. No. 4,469,195 of Sartain shows a ladder attachment including a pair or rotatable jaws or horns which are normally biased closed by a spring, which is interconnected between the horns. Extensions of the horns force the horns apart when pressed against a tree. The ladder is released and lowered by a pull rope, which is attached at one end to the horns and extends to the bottom of the ladder to be pulled by an operator.
Each of the above described ladders has certain defects, which affect the ease of erecting a ladder and the possible accidental release of the upper portion of the ladder from the pole or the like. Each of the German and Sartain patents has arms, which are spring biased to an open position to receive the pole. To close their arms and thus secure the ladder to the pole, it is necessary while balancing the ladder in its upright position to pull at the same time a cord or rope attached to those arms, thus bringing their arms together about the pole and securing the upper portion of the ladder to the pole. The Henson ladder has the advantage of permitting it to be easily erected by engaging his arms with the pole, thus moving the arms to a closed, holding position about the pole. However, both the Henson and Sartain ladders may be accidentally released from their poles. Only a spring serves to hold their arms together about their poles; a sudden motion by a person at the ladder top could readily overcome the retaining action of the springs on their arms and accidentally release the ladder from the pole. The German patent uses a detente to prevent its reel from being unwound and the arms released. However, that detente could be accidentally kicked or otherwise accidentally dislodged from the reel, while a person is at the top of the ladder.